I think you need to check your chassis info .
Workhorse chassis in 2004 , would be a W-22 or W-24.
I'd think that Brazel's RV would be a good source for anything Workhorse related .
Link to their web site is in the Vendors drop box , to the right of your " Users CP ".
EDIT : A quick check on the web site, gives a P/N ; w800061 and an out of stock notice .
Perhaps Johnny's AC Parts Warehouse, could help.